This invention relates to a process of upgrading low rank coal.
The amount of low rank coal deposits is estimated to be comparable to that of high rank coal. However, low rank coal is not widely utilized not only because of its low calorific value but also because of its difficulty in transportation and storage attributed to its high water content and spontaneous ignition when dried. A lot of processes have thus far been proposed to upgrade such low rank coal. One known upgrading method is a steam drying method in which a raw coal feed is heated at 120-130xc2x0 C. with steam. Another method is a pyrolysis method in which low rank coal is placed on a grid and contacted with hot gas to pyrolyze the coal to obtain upgraded coal and tar. A K-Fuel method is also known in which a raw coal feed is dehydrated with steam at 260xc2x0 C. or less and then heated at 340-430xc2x0 C. to obtain upgraded coal into which tar produced in situ is impregnated. The upgraded coal does not have a high calorific value.
The conventional processes have the following problems:
(a) upgraded coal still shows spontaneous ignition property and thus is ill-suited for storage and transportation;
(b) the upgraded coal has a significant oxygen content, which gives rise to a problem of CO2 generation;
(c) the oil product has a low content of valuable light fractions such as benzene, toluene, xylene, phenol and cresol.
It is, therefore, an object of the present invention to provide a process of upgrading low rank coal, which has solved the above-described problems.
Another object of the present invention is to provide a process capable of producing upgraded coal which does not have spontaneous ignition property and which is suitable for transportation and storage.
It is a further object of the present invention to provide an industrially advantageous process which can produce upgraded coal with a high calorific value and a reduced oxygen content but without spontaneous ignition property, and which can produce valuable light oils such as benzene, toluene, xylene, phenol and cresol.
In accomplishing the foregoing objects, the present invention provides a process of upgrading low rank coal, comprising subjecting the low rank coal to pyrolysis in a liquid phase at a temperature of 400-450xc2x0 C.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments to follow.
In the process according to the present invention, a low rank coal is pyrolyzed in a liquid phase at a temperature of 400-450xc2x0 C. to produce a upgraded coal and a light oil.
The low rank coal used in the present invention generally has a rank of coalification of 80% or less. Illustrative of the low rank coal are sub-bituminous coal, brown coal, lignite and peat.
For reasons of facilitating the pyrolysis, it is preferred that the low rank coal be ground to have a particle size of 100 mm or less, more preferably 50 mm or less. If necessary, the low rank coal feed is dried to remove water.
The low rank coal feed is preferably dispersed in the form of a slurry in an organic solvent. The organic solvent is generally used in an amount of 0.5-2 parts by weight, preferably 1-1.5 parts by weight per part by weight of the low rank coal.
Any organic solvent may be used as long as the coal feed can be slurried therein. Illustrative of suitable organic solvents are organic solvent is selected from the group consisting of paraffins, paraffinic kerosene oil, gas oil, lubricating oil fractions and waste oil. The use of a paraffin such as trans-decalin is especially preferred for reasons of high pyrolysis efficiency and easiness in solid-liquid separation of the pyrolyzed products.
The coal slurry is then pyrolyzed. It is important that the pyrolysis should be performed at a temperature of 400-450xc2x0 C., preferably 420-440xc2x0 C. When the temperature is less than 400xc2x0 C., the pyrolysis does not proceed sufficiently so that the yield of light oil fraction is very low. The term xe2x80x9clight oil fractionxe2x80x9d herein is intended to refer to a fraction having a boiling point in the range of 60-220xc2x0 C. Such a fraction include aromatic hydrocarbons such as benzene, toluene and xylene and oxygen-containing compounds such as phenol and cresol.
To high a pyrolysis temperature in excess of 450xc2x0 C. is undesirable because the pyrolysis proceeds excessively so that the liquid phase including the solvent decomposes into gaseous products.
The pyrolysis may be carried out in a closed reactor such as an autoclave which is preferably maintained in an oxygen-free atmosphere such as in a nitrogen atmosphere. The pyrolysis pressure is not specifically limited but is generally in the range of 0.2-7 MPa, preferably 0.5-5 MPa.
If desired, the pyrolysis may be performed in the presence of a catalyst such as a desulfurization catalyst or a denitrification catalyst. Illustrative of such catalysts are iron sulfide, nickel-molybdenum supported on alumina, cobalt-molybdenum supported on alumina and a natural mineral catalyst containing, for example, cobalt and/or nickel.